Compact bridge circuit measures sensitive and accurate conductance of low-conductivity electrolyte solutions. The bridge utilizes a phase sensitive detector to obtain a linear deflection of the null indicator relative to the measured conductance

Schmidt, K.

English

NASA Technical Reports Server

115V 
AC 
August 1967
	 Brief 67-10294 
•
toy C
AEC—NASA TECH BRIEF 
AEC-NASA Tech Briefs describe innovations resulting from the research and development program 
of the U.S. AEC or from AEC-NASA interagency efforts. They are issued to encourage commercial 
application. Tech Briefs are published by NASA and may be purchased, at 15 cents each, from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Sensitive Bridge Circuit Measures Conductance of 
Low-Conductivity Electrolyte Solutions 
The problem.: 
To develop a circuit that permits sensitive and 
accurate conductance measurements of low-conduc-
tivity electrolyte solutions. The deflection of the null 
indicator near the point of balance must depend 
linearly on the resistance or conductance to be meas-
ured in order to facilitate conductometric titrations. 
The measured conductance must not depend appreci-
ably on the reactive component (lead capacities etc.) 
of the system to be measured. 
The solution: 
• A compact bridge circuit that uses a 25-ohm poten-
tiometer to achieve a null-point sensitivity of 10-4 
or less, and an accuracy of approximately 0.2% when
the tangent of the phase angle between the unbalance 
and reference voltages is zero. The bridge utilizes a 
phase sensitive detector to obtain a linear deflection of 
the null indicator relative to the measured conduc-
tance and, within certain limits, a negligible influence 
of the phase angle 0 on the .point of balance. A phase 
angle with tan =0.15 causes an error of only 0.8%. 
The bridge circuit is operated from a 115 volt, 60 Hz 
source and provides four conductance measurement 
ranges: 0 to 1tmh6, 0 to l0tmho, 0 to 100 zmho, and 
0 to I mmho. 
How it's done: 
The bridge is balanced by a 25-ohm potentiometer 
that forms part of a variable ratio arm. The approxi-
mate balance condition is obtained for the imho 
(continued overleaf) 
This doument was prepared under the sponsorship of the Atomic 
Energy Commission and/or the National Aeronautics and Space 
Administration. Neither the United States Government nor any per-
son acting on behalf of the United States Government assumes any
liability resulting from the use of the information contained in this 
document, or warrants that the use of any information, apparatus, 
method, or process disclosed in this document may not infringe pri-
vately owned rights.	 -
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meter ranges when Gx=aGs (G= lOaGs for the 
mmho range), where G and Os are the unknown and 
known conductances respectively, and a is the fraction 
of the total potentiometer resistance between the 
sliding contact and the grounded end of the slidewire. 
The unbalance voltage is amplified, substantially 
free of phase shift, by transistor amplifier network 
Tr l—Tr 4, and applied to the phase sensitive detector 
Tr 5 Tr 6. The phase sensitive detector acts as a ref-
erence voltage operated switch, producing a square-
wave function reference signal. The unbalance signal is 
therefore multiplied by the square-wave function, 
presented as a dc component of the product, and 
indicated on the microammeter. The meter deflection 
is proportional to the product of the absolute value 
of the unbalance voltage, and the cosine of the phase 
angle between the unbalance and reference voltages. 
Notes: 
1. The bridge circuit performance was tested by con-
necting a 1 megohm resistor to the unknown con-
ductance terminals, paralleled by a variable air 
dielectric capacitor. The bridge was then balanced 
in the 0- to l-tmho range for various settings of 
the capacitor.
Additional details are contained in Review of 
Scientific Instruments 37, p. 671-672, May 196. 
Inquiries concerning this innovation may be di-
rected to:
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B67-10294
Source: K. Schmidt 
Chemistry Division 
(ARG- 147) 
Patent status: 
Inquiries about obtaining rights for commercial 
use of this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Case Avenue 
Argonne, Illinois 60439 
Brief 67-10294	 .	 Category 01


Sensitive bridge circuit measures conductance of low-conductivity electrolyte solutions

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